The medical proteins or industrial enzymes useful for humans, which could only be obtained in a trace amount from the natural state in the past, could be mass-produced by the development of recombinant DNA technology. For example, E. coli cells have been most widely used as host cells for producing large amounts of such useful proteins, and useful recombinant proteins, including hormones such as insulin and β-endorphin, and immunomodulators such as interferon, have been produced by E. coli. 
However, there is a limit to the production of either glycoproteins that require post-translational modification such as glycosylation to have activity, or proteins having a very large and complex structure. Furthermore, when a useful protein is expressed in yeast, an insoluble inclusion body protein is formed which lost its activity by various mechanisms without being completely is formed. Although this insoluble protein may be easily isolated in an initial stage to provide a highly pure protein in some cases, it lacks activity as the protein. For this reason, complex and costly denaturation and refolding processes are required to obtain a biologically active soluble protein from the insoluble protein. Thus, there has been increasing interest in a method for producing a large amount of a target protein as a form of secretion.
Meanwhile, interleukin-2 consists of 153 amino acids and is produced mainly by T cells expressing the surface antigen CD4. Transformed T cells, B cells, lymphocytic cancer cells, LAK cells and NK cells also secrete interleukin-2. It is known that the production of interleukin-2 is induced by mitogen- or allergen-mediated activation of T cells, and several kinds of secondary stimulations are required to maximize the production of interleukin-2, but resting cells cannot produce interleukin-2. It has been reported that interleukin-2 and its receptor are associated with many disease However, studies on the molecular characteristics of interleukin-2 and its receptor have been very limited, because they are obtained in limited amounts.
For example, many methods have been studied to increase immunity against cancer by administration of functional interleukin-2 gene, and thus studies on interleukin-2 and the demand for interleukin-2 as a therapeutic agent have continued to increase. However, technology for producing a large amount of interleukin-2 is still insufficient.
Under this background, it is necessary to develop a gene expression system for mass production of interleukin-2 using various expression systems.